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ss7ja [257]
3 years ago
10

Explain why the precise length target DNA sequence doesn't get amplified until the third cycle. You may need to use additional p

aper and a drawing to explain your answer.

Biology
1 answer:
shusha [124]3 years ago
3 0

When you start the PCR amplification reaction, you have a template which has sequences where the primers will anneal. The forward primer anneals in the sense strand and the reverse one in the complementary sequence (see attached figure).

After the first PCR cycle, two types of fragments are obtained. First those which come from tha 5'-3' sequence, that are 3'-5' and will anneal the reverse primer in the following cycle. This fragments have the correct 3' end, but the 5' includes part of the template that doesn't correspond to our target (see attached figure)

Second, those fragments that come from the 3'-5' sequence, that are 5'-3' and will anneal the forward primer. This fragments also have the correct 3' end, but the 5' includes part of the template that doesn't correspond to our target (see attached figure).

Now, during the second cycle, are produced the first fragments that have the precise length of the sequence that we want to amplify (see attached figure).

So, it's not until the third cycle that the reaction starts amplifying exactly the sequence of desired length.

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Note 2: Unbalanced forces are the forces that cause change in movement and motion. When two unbalanced forces are exerted in opposite directions, the overall force (or net force) is the difference between the two forces and the direction is in favor for the larger force. 
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Q22: The two forces on the left and right side are balanced given that their force is equal in size and they are in opposite directions. The two forces northwards and southwards are unbalanced forces since they are not equal in size but are opposite in direction. Recall in note 2 when I given the formula to find the net force for unbalanced forces. Its the difference (subtraction) between the bigger force and smaller force. 5 - 2 = 3. Therefore the net force for question 22 is 3.The direction is in the bigger force's direction, therefore southwards. 


Q24: Is a balanced force. Therefore the net force is 0. I don't think there is technically any direction per say. 
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This case is not overturned until 1954, in which the case Brown vs. Board reverses this idea and declares that separate but equal facilities violates the 14th amednment rights of African-American citizens.

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